The present invention relates to a polymer electrolyte battery, particularly to a flat polymer electrolyte battery comprising a thermally unitary laminated structure composed of electrode films or sheets and separator films or sheets, the electrodes and separators containing a polymer capable of absorbing and retaining a nonaqueous electrolyte.
With the recent development of compact, thin and lightweight portable appliances, there has been a strong demand for a compact, thin and lightweight battery as a power source therefor. Lightweight lithium ion batteries affording a high energy density have already been realized, much contributing to down-sizing and weight reduction of portable phones and portable personal computers. However, there is still an ever-increasing serious demand for further minimized lightweight portable appliances. However, conventional lithium ion batteries realizing a short distance between positive and negative electrodes and a satisfactory contact between the electrode and a separator by tightly securing a laminate of the positive and negative electrodes and the separator inside a rigid and firm metal jacket case have the following drawback: A certain thickness is required for the metal jacket case in order to secure its strength, which limits realization of a thinner lithium ion battery.
One noted method for realizing a thin lithium ion battery is lithium polymer secondary batteries including a polymeric material as the electrolyte. Among them, a battery system wherein an electrolyte layer and electrodes are unitarily bound to each other is effective in realizing a thin battery system, because-it can provide a satisfactory contact between the electrolyte layer and the electrodes without need of the above-mentioned rigid and firm metal jacket case. Examples of a battery comprising a unitary bound structure of an electrolyte layer and electrodes can be found in the U.S. Pat. Nos. 4,830,939 and 5,478,668.
The U.S. Pat. No. 4,830,939 discloses a lithium battery wherein a mixed solution of a monomer and an electrolyte is applied onto the negative electrode made of metallic lithium or the positive electrode, which is then irradiated with ultraviolet ray or electron beam to polymerize the monomer and eventually form a solid polymer electrolyte. Since the layer of the electrolyte is formed along the fine uneven surface of the electrode, the contact between the electrode and the electrolyte layer becomes satisfactory without securing the one to the other.
The U.S. Pat. No. 5,478,668, on the other hand, discloses a copolymer (hereinafter referred to as xe2x80x9cP(VDF/HFPxe2x80x9d) of vinylidene fluoride (hereinafter referred to as xe2x80x9cVDFxe2x80x9d) and hexafluoropropylene (hereinafter referred to as xe2x80x9cHFPxe2x80x9d) as a polymer material. A positive electrode, a negative electrode and a polymer separator are formed first and then the separator is unitarily laminated to the positive or the negative electrode.
The use of such unitary bound sheet-like structure of an electrode and an electrolyte layer as the power generating element as disclosed in the above-mentioned two U.S. patents can give a thin rechargeable battery even when it is housed in a jacket case of a thin and flexible laminate sheet.
In order to obtain a battery which can give an operational battery capacity, this type of battery was initially formed in multiplexed-cell structure by simply piling a multiplicity of cells, each cell being formed, using a pair of positive and negative current collectors, by placing a positive electrode sheet and a negative electrode sheet to face each other, with a polymer separator being placed therebetween. The positive electrode sheet comprises a current collector and a layer of positive active material mixture disposed on one surface of current collector and the negative electrode sheet comprises a current collector and a layer of negative active material mixture disposed on one surface of the current collector. This structure, however, produces a drawback that the surface of current collector not in contact with the active material mixture fails to participate in the event of discharge, which in turn increases the percentage of the current collectors in the battery volume, compared to the conventional lithium ion battery.
In order to solve the above-mentioned problems, there is a recently proposed structure of a battery in the Japanese Laid-Open Patent Publication Hei 10-189053 such that a negative electrode whose porous current collector has a layer of negative active material mixture on both surfaces is sandwiched between positive electrodes whose porous current collector has a layer of positive active material mixture on one surface or both surfaces, with a polymer separator interposed therebetween. This structure facilitates realization of a thin and lightweight battery of high energy density while retaining satisfactory characteristics as a battery.
Specific production process of such polymer electrolyte battery is shown below:
First, a paste is prepared from a mixture of an electrode active material powder and a conductive agent powder added with an organic solvent solution of a polymer and dibutyl phthalate as a pore forming material. The resultant paste is applied onto both surfaces of a porous current collector for the negative electrode and one or both surfaces of another porous current collector for the positive electrode. Those two electrode sheets are then dried to evaporate the organic solvent, which gives a positive electrode sheet and a negative electrode sheet. The positive and negative electrode sheets thus obtained and polymer separator sheets containing a pore forming material are laminated alternately in the order of positive electrode sheet, separator, negative electrode sheet, separator and positive electrode sheet. The resultant laminate is pressed under heat, which gives a unitary, laminated sheet for use as a battery element. The battery element sheet is then immersed in, for example, an extracting solvent diethyl ether for removing the pore forming material from the sheet. After the battery element sheet becomes porous enough and has pores by this treatment, a nonaqueous electrolyte is allowed to impregnate into the pores of the electrode sheet and the polymer separator sheet.
In the polymer electrolyte battery thus obtained, the capacity density depends on the porosity and the ratio of polymer of the electrodes. This means that if the loading or packing density of electrode active material is increased by rolling the electrode sheet after applying the above-noted paste onto the current collector to reduce the degree of porosity, the electrolyte would not enter the electrode sufficiently. As a result, the utilization of electrode active material decreases. To the contrary, if the total spatial volume inside the electrode is increased by lowering the loading density of electrode active material and the porosity is increased, the electrolyte enters the electrode sufficiently. This increases the utilization of electrode active material but reduces the absolute volume of active material. Higher percentages of polymer in the electrode result in relative reductions of the volume of active material and lower percentages result in impaired electrode strength. When the percentage of polymer in the electrode is high, the polymer becomes rubbery upon stirring to make it into paste, which disturbs sufficient rolling of the electrode. Furthermore, the use of a lathe sheet as the porous current collector may not increase the loading density of electrode active material, because the current collector is stretched together with the electrode active material mixture and is even torn or pulled off during rolling. To the contrary, a low percentage of polymer in the paste interferes with thermally unitarily laminating the electrode sheets and separators, forming a gap between the electrode and the separator and elevating the internal resistance of the resultant battery. Therefore, stable performance cannot be expected for such battery. Based on the above considerations, the adequate percentage of polymer in the layer of electrode active material mixture has been conventionally 20 wt % or so.
In view of the above-mentioned problems, the object of the present invention is to provide a thin and lightweight polymer electrolyte battery of high capacity density by optimizing the porosity of electrode sheet using a certain range of concentration of polymer in the layer of electrode active material mixture and/or by optimizing the polymer content by regulating to a fixed extent rolling operation for adjusting the porosity of electrode sheet, thereby increasing the loading density of electrode active material mixture.
The present invention provides a polymer electrolyte battery including:
a nonaqueous electrolyte,
positive and negative electrode films or sheets each of which comprises a porous current collector and a layer of active material mixture disposed on both surfaces of the porous current collector, the layer of active material mixture containing a polymer capable of absorbing and retaining the electrolyte, and
porous separator films of a polymer capable of absorbing and retaining the electrolyte, the electrolyte being retained in the positive and negative electrodes and the separators,
wherein the positive electrodes are unitary laminated on the negative electrode with the separator being placed on both surfaces of the negative electrode to form a unitary laminated battery element sheet, and wherein the separator and the layer of active material mixture of the positive and negative electrodes have a porosity in a range of 30 to 60%.
For this battery, a preferred porosity of the layer of active material mixture is in a range of 35 to 55% for the positive electrode and in a range of 35 to 45% for the negative electrode. A preferred porosity of the separator is in a range of 50 to 55%.
The porosity of the layer of active material mixture is represented as follows:
(Total spatial volume of the layer of active material mixture)/(total volume of the layer of active material mixture)xc3x97100 (%).
The present invention also provides a polymer electrolyte battery comprising a nonaqueous electrolyte, positive and negative electrode films or sheets whose porous current collector has a layer of active material mixture containing a polymer on both surfaces, and porous separators of a polymer, each polymer being capable of absorbing and retaining the electrolyte, the electrolyte being retained in the positive and negative electrodes and the separators, wherein the positive electrodes are opposed to the negative electrode with the separator being placed on both surfaces of the negative electrode to form a unitary laminated battery element sheet, and wherein the layer of active material mixture contains the polymer in a range of 5 to 10 wt % for the positive electrode and in a range of 7 to 16 wt % for the negative electrode.
For this battery, it is more preferable that the layer of active material mixture contains the polymer in a range of 6 to 8 wt % for the positive electrode and in a range of 8 to 15% for the negative electrode.
The present invention provides a further polymer electrolyte battery whose polymer content in the layer of active material mixture is in a range of 5 to 10 wt % for the positive electrode and in a range of 7 to 16 wt % for the negative electrode and whose separator and layer of active material mixture have a porosity in a range of 35 to 55%.
For this battery, it is preferable that the layer of active material mixture has a porosity in a range of 35 to 55% for the positive electrode and a porosity in a range of 35 to 45% for the negative electrode, and contains the polymer in a range of 6 to 8 wt % for the positive electrode and in a range of 8 to 15 wt % for the negative electrode.